Several tone signal detectors have been proposed for detecting the presence of predetermined tone signals. One of such detectors, which is disclosed in the inventor's copending patent application, Ser. No. 854,868, filed Nov. 25, 1977, includes a waveform converting circuit for converting incoming tone signals into pulses which are applied, as gating pulses, to a suitable gate circuit. The gate circuit responds to the gating pulse and selectively permits clock pulses having a high repetition rate to be applied to a counter. The counter yields a tone signal detecting pulse only upon counting the clock pulses up to a predetermined number during a predetermined time interval. The detector also detects tone signals each having a predetermined period for the purpose of noise suppression.
The tone signal detector, however, has encountered a problem: a threshold in the waveform converting circuit is liable to vary, resulting in variations of duty factor of the gating pulse, so that the detector is subject to failure in predetermined tone signals.
For a better understanding of this invention, reference is made to FIGS. 1-4 of the accompanying drawings, wherein FIGS. 1 and 2 are schematic illustrations of conventional tone signal detectors and FIGS. 3 and 4 are time charts illustrating proper and improper operations, respectively, of the detectors of FIGS. 1 and 2.
A difference between the circuit configurations of FIGS. 1 and 2 is that one input terminal of AND gate 5 of the former is coupled to the Q output of a flip-flop 3, not to the Q output as in the latter. To begin with, it is assumed that a waveform converting circuit 2 maintains its threshold at a proper level, so that the flip-flop 3 produces complementary pulses each having proper duty factors as shown in FIG. 3.
A tone signal is applied, through a band-pass filter 1, to the waveform converting circuit 2 (for example, a voltage comparator comprising a differential amplifier 7 and a reference voltage source 8). The waveform converting circuit 2 compares the voltage of the incoming tone signal with the reference voltage to produce a rectangular wave signal. The output of the waveform converting circuit 2 is fed to the D input of the flip-flop 3 (a D type flip-flop, for example). The flip-flop 3 also receives, at its C input, clock pulses "c" from a clock pulse generator 4 to synchronize the output signal of the waveform converting circuit 2 with the clock pulses "c". Pulses "a" and "b" developed, respectively, at a Q output and Q output (FIGS. 1 and 2) are then fed as gating pulses to the AND gate 5. The AND gate 5 responds to the gating pulse and selectively allows the clock pulses "c" to be applied to the next stage, viz., a counter (in this case, a ternary counter) 6. The counter 6 counts the clock pulses and produces a detecting pulse "d" or "e" at its output OUT every three pulses counted. The pulse "b" (FIG. 1) or "a" (FIG. 2) resets the counter 6.
Under the above assumption, either of the circuit arrangements of FIGS. 1 and 2 is practical. However, when the pulse developed at the Q or Q output varies in its duty factor as shown in FIG. 4 due to the undesirable changes of the threshold, the detector of FIG. 1 or 2 cannot properly detect the presence of predetermined tone signals.
In the above, when the output of the waveform converting circuit 2 is not necessarily synchronized with the clock pulses "c", the flip-flop 3 can be omitted, wherein the inverting or non-inverting output of the differential amplifier 7 is directly connected to the AND gate 5.
It is therefore a primary object of this invention to provide an improved tone signal detector which yields detecting pulses without being adversely affected by threshold variations in the waveform converting circuit.
A further object of this invention is to provide an improved tone signal detector which examines frequency deviation of incoming tone signals and detecting the presence of tone signals each having a predetermined period.